Vehicle fuel tanks normally and continually produce fuel vapors, often called the diurnal vapors or diurnal loss, which collect in a vapor dome at the top of the tank. If unrelieved, they could potentially over pressurize the tank. Diurnal vapors are routinely vented to a storage canister from which they are later purged by engine manifold vacuum and burned. The opening from the tank vapor dome to the canister is typically highly restricted, so as not to encourage vapor formation.
A much higher volume of vapor is displaced from the fuel tank when it is filled, known as the fuel fill vapors or fill loss. The fill loss must be allowed to exit the tank somehow, or the tank pressure would quickly rise so high that liquid fuel would back up the fuel tank filler pipe and shut off the fuel nozzle. Historically, fill losses have simply been vented to atmosphere. Many new fuel systems propose to recover the fill losses, as well, through some kind of tank vapor vent valve that routes the displaced vapor to the same canister. The exit for such a tank vapor vent valve must be considerably larger than the restricted opening in the diurnal loss line, since a high volume of vapor is expelled in a very short time. The larger exit of the tank vapor vent valve should not be exposed to the vapor dome except during fill, so as not to encourage diurnal vapor formation.
This consideration has led to many different proposed valve designs in which the valve is opened by an event that occurs only during fuel fill, such as the removal of the gas cap, or the insertion of the fuel nozzle. Otherwise, the valve is closed. Most of these designs also locate all the valve hardware near the end of the filler neck, so as to be close to the gas cap or filler nozzle. Other designs have proposed to locate the valve hardware remote from the filler neck, but still activated by the gas cap removal or nozzle insertion. These typically use a wire or other moving part stretching from the end of the filler neck to the remote valve.